Although my nickname is Imtiaz, everyone in class calls me “The Scientist.” Maybe the reason is that, whether or not I actually know more than them, at least I try to know more. Perhaps it’s this thirst for knowledge that causes some differences of opinion between me and them, making me the class’s resident scientist. Another thing about me is that I usually spend more time in the library than outside my classroom at school. That’s where I get to talk to many people, most of whom are much younger than I am. They ask me all sorts of questions that are on their minds and want to know about many things, and I try my best to help them. It almost feels like I’ve set up my own little school.
So, one day, I ended up having a conversation about gravity with a student from class eight. On one hand, he only knew about Newtonian gravity, and on the other hand, school ended early that day and we had plenty of time. So, I decided to give him a bit of a modern explanation of gravity, as much as possible. Let’s see if our conversation helps you too. Happy reading.
Student: Bhaiya, which chapter of physics do you like studying the most? For me, it’s the chapter on gravity.
Me: Is that so? I also love studying or discussing any topic related to gravity. So, what’s your idea of gravity—what do you actually know about it?
Student: I know what’s in our textbook. For example, if we place two objects in space, they will pull each other towards themselves—that is, they attract each other. This attraction is gravity.
Me: Oh! So you only know about Newton’s theory of gravity?
Student: Why? Are there many theories about gravity?
Me: Yes. There are actually two main theories about gravity. One is Newton’s theory of gravity, which you know. The other is Einstein’s General Theory of Relativity. In fact, before 1905, there were no problems with Newton’s theory. But in 1905, when Einstein stated in his Special Theory of Relativity that the speed of light is the highest speed in the universe—that nothing can surpass it—problems began to arise with Newton’s theory.
Student: What kind of problems, exactly?
Me: To understand this, let’s try a bit of imagination. We know that it takes about 8 minutes and 20 seconds for light to reach the Earth from the Sun. Now, suppose the Sun suddenly disappeared or was destroyed for some reason. In that case, it would still take about 8 minutes for the light from the Sun to stop reaching Earth. But according to Newton’s theory, as soon as the Sun disappears, its gravitational pull on Earth would also stop instantly. And that’s where the main issue is. If it takes about 8 minutes for light to travel that distance, gravity, according to Newton, seems to act instantaneously—regardless of distance! That’s a real problem because Einstein’s theory told us that nothing, not even gravity, can travel faster than light. But if Newton’s theory were correct, gravity could easily outpace light, as gravity would need no time at all, while light needs 8 minutes! So, clearly, Newton’s theory isn’t fully correct. There are also other technical errors (I’m not mentioning them all; just giving you an idea). Because of these issues, Einstein started working on addressing the limitations and problems of Newton’s theory. As a result, in 1915, he developed the General Theory of Relativity. And that’s the history behind the discovery of General Relativity.
Student: I think I get it. So according to the General Theory of Relativity, what exactly is gravity?
Me: To understand that, you first need to know what space-time is. Previously, people thought our universe was three-dimensional; meaning, it had only length, width, and height. But Einstein said the universe is four-dimensional—including length, width, height, and time. The three dimensions of space combined with the one of time together make up what we call space-time. Simply put, every part of this four-dimensional universe is space-time.
Student: Umm, okay. No problem so far. I think I get the basic idea.
Me: That’s enough for now. So, when we place an object in space, it bends the space-time around it. The kind of distortion in space-time caused by an object is called space-time curvature. According to the General Theory of Relativity, the effect of this curvature on an object is what we experience as gravity—and it’s not actually a force of attraction in itself. When you put an object in space, it bends not only the space around it but also time. Because of this, time runs slower in regions with stronger gravity; this is called gravitational time dilation. Similarly, the curvature of space-time due to an object also bends the space around it. Now, if an object can create a sufficiently strong curvature, it bends the space around it so much that nothing, not even light, can escape. That’s because space becomes so warped around the object that anything entering gets trapped, always returning to the same place, and can never escape—like what happens with a black hole. A black hole bends the space around it so much that nothing that enters can come out.
Student: We learned that you can’t make an object move without applying a force. If gravity isn’t a force of attraction, then how does the Sun make the Earth continuously orbit around it without exerting any force?
Me: Hmm. You’ve probably heard of Newton’s First Law of Motion. It says that an object in motion will continue moving in a straight line at a constant speed unless acted on by an external force. Now, we know Earth is moving; it should have been moving in a straight line. But since the space of the solar system is curved due to the Sun’s space-time curvature, the Earth’s path is also curved. As per General Relativity, gravity is not a force, so there’s no external force acting on the Earth. The force that initially brought Earth into the Sun’s space-time curvature in the past is still keeping it there, because there’s no external force now to undo it. When we say the Earth orbits the Sun because of the Sun’s gravity, that’s not strictly correct. Earth orbits because of the effect of the Sun’s space-time curvature. Now you see how, without applying any force, gravity compels an object to orbit around another.
Student: I see. So if an object enters the space-time curvature of another, it will orbit around it due to the curvature—even though there’s no external force acting on it. The force that initially put it into that path is what keeps it moving, and since space itself is curved, the object is forced to move around the other object. So, the effect of space-time curvature on an object is gravity, right?
Me: Exactly right. One more thing—the more massive an object is, the stronger its space-time curvature will be. For one object to make another orbit it, the first object needs to have strong enough space-time curvature.
Student: Does the effect of space-time curvature work on everything?
Me: Yes. Regardless of whether or not something has mass, everything is affected by space-time curvature. For example, light. Light has no mass, yet it, too, gets its path bent by space-time. Scientists have observed that when the light from other stars passes near the Sun, it is slightly bent because of the Sun’s space-time curvature. This bending of light’s path due to the curvature of space-time is called gravitational lensing. Because of gravitational lensing, we can’t see the actual position of an object in space; that is, the positions we observe are not the true positions of the objects!
Student: But we’ve read that light always moves in straight lines. How, then, can light go on a curved path?
Me: Well, the space around the Sun gets bent due to its space-time curvature. So, when light passes near the Sun, it has no choice but to travel through that curved space. That’s why it is forced to follow a bent or curved path. Make sense?
Student: Yeah, I think I get the whole concept now. It’s really interesting.
Me: Oh, it’s gotten pretty late; I need to head home now. I’m off. By the way, the discussion about General Relativity isn’t done yet—there’s still a lot more left. Maybe we’ll talk more about it some other day.
Student: Alright, Bhaiya, let’s talk again later.
Last words: I hope those who have read through this carefully now have quite a good basic understanding of General Relativity. But this isn’t the end—there’s still plenty more to learn about it.
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